A large chunk of these seem to be release critical, especially those
which depend on python python2.6 (libpython2.6) and python2.5.

On the contrary, these are safe to ignore in most cases. Such
dependencies are typically autogenerated because Python modules are
(unnecessarily) linked with libpythonX.Y. While this is wasteful, it's
harmless, and it's certainly nothing that warrants an RC bug.

Please don't just binNMU, this was apparently already done in the past
without watching the build results.

Huh.

grass 6.4.0~rc6+42329-2

In sid doesn't depend on python2.5 on any release architecture AFAICS.